The proposed research is a structural study of the acrosomal process of Limulus sperm. The process consists of a bundle of actin-containing filaments, which are coiled about the base of the sperm. On contact of the sperm and egg, the bundle uncoils, extending outward toward the egg. This system exemplifies a novel mechanism for the action of actin. The dramatic extension of the acrosomal bundle is driven by a change in twist of its actin-containing filaments. The object of the study is to determine how the change in twist is used to generate the force and motion. The process exists in three distinct structural states: a coil form; a false discharge form, which is a transient state induced by contact of the sperm with sea water; and the extended or true discharge form. The objective of the research is to determine the structures of the three states and the pathway of structural change between them. We propose to examine the three forms of the bundle by the combined techniques of electron microscopy and optical diffraction. With the results, we can determine the changes in the structure and arrangement of filaments and thus deduce the mechanism of the reaction. It is likely that other actins have the capability to change twist. Such structures can do work, in particular in bundles, where the forces from individual filaments are coupled together. Actin bundles are common structures in non-muscle cells (for example microvillae contain such bundles). The bundle in Limulus is an excellent model system to develop with a view toward application to other structures.